Risk assessment studies performed by the United States Environmental Protection Agency (EPA) for hazardous air pollutants (HAPS) emitted from coal-fired power plants have identified mercury as the HAP of most concern (USEPA, 1997). US coals typically contain 10-120 ppbw mercury. It is estimated that coal-fired power plants emit approximately 48 tons of mercury annually, or about one-third of the total US anthropogenic mercury emissions. Mercury released from coal during combustion exists as Hg0 at high furnace temperatures. After leaving the high-temperature furnace environment, homogeneous as well as heterogeneous oxidations convert a portion of the Hg0 to Hg2+.
Two general approaches based on the use of sorbents have been pursued for the removal of mercury at particulate control devices. One approach uses upstream (furnace) injection of the sorbent. In a study on emissions from burning a blend of Ohio 5, 6 and 7 coal, upstream injection with two sorbents (lime and carbon) was demonstrated to remove between 41% and 53% of total (particulate+vapor) mercury, which implies that ⅓ or less of the vapor-phase mercury was removed. The second approach has been to use sorbents at various locations downstream of the boiler. A variety of sorbents including carbon, fly ash, and minerals have been studied, and fixed-bed, fluidized-bed and direct-injection contacting schemes have been proposed. Carbon and chemically modified carbons have been extensively researched. The chemically modified carbons are impregnated with chlorine, sulfur or iodine to improve adsorption characteristic. Problems with very low capacity, poor mixing and low thermal stability make the carbon injection approach very expensive. The US Department of Energy (DOE) estimates the projected cost for 90% mercury control to be between $25,000 to $70,000 per lb Hg removed. Others examined various sorbents including chemically promoted activated carbons and metal oxides and sulfides for their capability to remove elemental mercury from flue gases. The results showed that activated carbon treated with hydrochloric acid is most promising with an adsorption capacity of 4.0 mg Hg/g. Molybdenum disulfide (MoS2) displayed a large capacity (8.8 mg Hg/g) for elemental mercury, but is known to be expensive and unstable at elevated temperatures.